1. Field of the Invention
The present invention relates to a solid polymer electrolyte and a preparation method therefor. More particularly, the present invention relates to a solid polymer electrolyte which is superior in ionic conductivity and mechanical strength, and a preparation method therefor. The solid polymer electrolyte according to the present invention is suitable for electrochemical devices such as batteries, particularly for secondary batteries of high energy density.
2. Description of the Prior Art
As electronic and information systems increasingly feature size reduction and portability, research and development is now being made on light-weight and high-voltage secondary batteries, among which metal lithium secondary batteries are promising power sources for these systems because of their light weight and high energy density. In general, such a lithium battery employs metal lithium as its negative electrode and a nonaqueous electrolytic solution containing a lithium salt as its electrolyte.
It is, however, known that dendrites (branching tree-like crystals) are generated on the metal lithium during repeated charge and discharge cycles when it uses metal lithium as negative electrode for lithium secondary battery, resulting in a short circuit within the battery and deterioration of the cycle characteristic of the battery.
Attention is now focused on lithium ion secondary batteries, which have already been put to practical use. Such a lithium ion secondary battery employs, instead of the metal lithium negative electrode, a negative electrode which comprises a host such as of a carbon material and lithium ions and utilizes an intercalation and deintercalation reaction of the lithium ions in the host. The lithium ion secondary battery generally has a lower theoretical negative electrode capacity than the metal lithium secondary battery, but is superior in the cycle characteristic and reliability.
In general, the lithium secondary batteries (including the metal lithium secondary batteries and the lithium ion secondary batteries) employ organic electrolytic solutions as their electrolytes. However, the use of such a liquid electrolyte imposes problems associated with the reliability of the battery, e.g., deterioration of the battery which may result from leakage of the electrolytic solution out of the battery, vaporization of a solvent of the electrolytic solution and dissolution of an electrode material in the electrolytic solution. Further, the organic electrolytic solution contains a flammable organic solvent and, hence, the leakage of the solvent may result in ignition.
There is a demand for a battery which employs a solid electrolyte composed of an inorganic material or a polymeric material and is free from the solution leakage out of the battery. Particularly, a solid electrolyte composed of a polymeric material (hereinafter referred to as "solid polymer electrolyte") has the advantages of relatively easy preparation, low costs and light weight. The solid polymer electrolyte is attractive because totally solid batteries featuring a smaller thickness and a shape variation can be provided by employing the solid polymer electrolyte.
The solid polymer electrolyte is highly safe, but has a lower ionic conductivity than the conventional organic electrolytic solutions. Exemplary polymers currently used for the solid polymer electrolyte include polyethers such as polyethylene oxide and polypropylene oxide. Since linear polymers such as polyethylene oxide and polypropylene oxide are crystalline polymers, a solid polymer electrolyte composed of such a polymer and an electrolytic salt has a satisfactory ionic conductivity at a high temperature, but a low ionic conductivity at a temperature not higher than ordinary temperature.
A known approach to the problem of the reduction in the ionic conductivity is to use an amorphous polymer obtained by cross-linking polyethylene oxide and/or polypropylene oxide into a graft structure or a network structure.
However, a solid polymer electrolyte composed of such an amorphous polymer still has a lower ionic conductivity than the organic electrolytic solution, and does not exhibit a satisfactory ionic conductivity at a temperature not higher than ordinary temperature.
To further improve the ionic conductivity, an organic solvent is added to the aforesaid ion conductive polymer to such an extent that exudation of the organic solvent does not occur, or the ion conductive polymer is formed into a thin film for reduction of the resistance of the entire ion conductor. A totally solid battery employing a solid polymer electrolyte thus obtained has a reduced internal resistance.
However, the solid polymer electrolyte composed of the polymer cross-linked into a network structure is superior in the ionic conductivity, but has a very low mechanical strength. Therefore, if the solid polymer electrolyte is employed for a battery, the solid polymer electrolyte may be damaged by pressure applied thereto during fabrication of the battery or during the charge and discharge process of the battery.
To solve this problem, various solid polymer electrolytes are proposed.
For example, Japanese Unexamined Patent Publication No. SHO 63(1988)-102104 proposes a composite solid polymer electrolyte obtained by impregnating an electrolytic polymer such as polyethylene oxide in a polymeric porous film such as of a polycarbonate or polyvinyl chloride. Japanese Unexamined Patent Publication No. HEI 8(1996)-148163 proposes a composite solid polymer electrolyte containing a powdery insulative material such as glass or a ceramic or a powdery ion conductive material dispersed therein. Further, Japanese Unexamined Patent Publications No. HEI6(1994)-140051 and No. HEI6(1994)-150941 propose solid polymer electrolytes composed of a polymer blend of polyvinyl alcohol and polyethylene oxide or a copolymer of vinyl alcohol and ethylene oxide.
However, these solid polymer electrolytes suffer the following drawbacks.
The composite solid polymer electrolyte (ion conductor) comprising the polymeric porous film and the polymeric electrolyte has a greater resistance because the content of the electrolyte component serving for ion conduction is reduced. Therefore, if the composite solid polymer electrolyte is employed for a battery, a reduction in the battery capacity and an increase in the internal resistance may result.
Further, the composite solid polymer electrolyte containing the powdery glass or ceramic dispersed therein has a high mechanical strength, but the preparation thereof is costly because an additional step for particle size classification of the particles is required for formation of a homogenous solid polymer electrolyte film.
The solid polymer electrolyte composed of the polymer blend of polyvinyl alcohol and polyethylene oxide or the copolymer of vinyl alcohol and ethylene oxide is excellent in the ionic conductivity and the mechanical strength. However, hydroxyl groups in polyvinyl alcohol are reactive with lithium. Therefore, if the solid polymer electrolyte is employed for a metal lithium battery or a lithium ion battery, the hydroxyl groups react with lithium, so that it is difficult to maintain polyvinyl alcohol stable in the battery. Therefore, an electrode containing the solid polymer electrolyte has problems associated with the stability and cycle characteristic.